Measurement of DC Electric Field Profile under Ion Flow Using Electric-Field-Induced Second-Harmonic Generation and Pockels Effect

Corona discharge during the operation of HVDC transmission lines generates ion flow around the conductors. The resulting electric fields may interfere with nearby communication equipment. Measuring these fields is essential for evaluating and mitigating such interference. However, traditional contact-based probes can disturb the field distribution and accumulate space charge on their surfaces, reducing measurement accuracy.

To address this issue, we employed a non-invasive measurement technique based on electric-field-induced second harmonic generation (E-FISHG) to spatially resolve DC electric field distributions under ion flow conditions. The experimental system uses an ion flow generator to produce a controlled DC field and steady ion flow. A pair of aluminum pipes—one of which is adjustable—forms an optical tunnel to confine the nonlinear interaction region and mitigate phase mismatch effects, thereby enhancing signal sensitivity. By varying the height of the laser relative to the electrode, we acquired second harmonic (SH) signals at various heights under different applied DC voltages. The resulting electric field profiles showed good agreement with the theoretical predictions derived from the space-charge-limited current (SCLC) model.

To further validate the measurement results obtained by E-FISHG, a supplementary experiment using the Pockels effect was conducted. A grounded shutter was employed to control the exposure of the Pockels crystal to the external electric field, preventing both premature polarization and surface charge accumulation. Additionally, an optical chopper and lock-in amplifier were used to extract weak signals. The electric field trends observed using the Pockels method were consistent with those obtained by E-FISHG, confirming the reliability of the measurement approach.